The present invention relates to a suspension pad for a resonant beam support system.
A resonant beam support system provides a vertical impact system, for example, to break pavement or prepare the ground for construction or other uses by tamping beneath a mobile vehicle. The vehicle includes a resonant beam supported at its nodes and excited at one end near its resonant frequency. A tool projects downwardly from the output end of the beam to break the pavement or tamp the ground. A large weight is superimposed over the forward node of the beam to counteract the reaction forces of the tool striking the underlying surface. The weight is suspended from the vehicle so that the reaction forces are not transmitted by the weight to the vehicle, thereby isolating the vehicle from the reaction forces.
U.S. Pat. No. 4,515,408 entitled "Counterweight Support For Resonantly Driven Tool" issued on May 7, 1985 describes a mechanism for positioning the weight relative to the resonant beam in a resonantly driven impact system, the disclosure of which is incorporated by reference. The weight is linked to the resonant beam at the node proximate the output end of the beam. A member is fastened to the weight and extends to a position adjacent the node of the beam proximate the input end of the beam. A recess is formed in the lower portion of the weight and includes a plate located therein. As the beam oscillates, pads constructed of a resilient, deformable material intermediate the plate and the walls of the recess provide a large bearing surface for the weight so that the weight can absorb large reaction forces transmitted by the tool through the beam. The resiliency of the pads allows for vibratory movement of the beam relative to the weight.
The incompressibility of the resilient, deformable pads results in a non-linear resistance to compression, i.e., a non-linear spring rate. This occurs because the fixed volume of the pad must expand laterally as it is compressed vertically. Thus, only deflections of small magnitude occur at the true spring rate as determined by the elastic modulus of a given rubber compound. However, further deflection is resisted by tensile forces in the mass as it expands in other directions. As a block of rubber is compressed between its top and bottom faces, these tensile forces increase progressively from the center to the outer lateral surfaces. The material at the center of the pad is latterly restrained by the entire outer mass. The center of the pad exhibits the greatest resistance to compression, or in other words, the greatest spring rate. The overall spring rate is equivalent to the summation of the component forces at each position.
Rapid repetition of compression and relaxation creates internal friction in the rubber, causing a buildup of heat. The amount of heat energy required to raise rubber one degree is equal to approximately one-third of the amount of heat energy required for increasing water temperature by the same amount. Rubber, being a poor conductor of heat, allows this buildup to continue to destruction of the pads. The buildup of heat also increases the volume of material from thermal expansion which can add to the compression and hasten failure. As the temperature of the rubber is raised, the elastic modulus is lowered. Because the resonant beam support system requires a constant, exact and consistent spring rate, solid rubber pads do not provide optimum concrete breaking action for the resonant beam support system or adequate life of the pads themselves.